Catalysis for Low-Temperature Fuel Cells

Due to their cost and scarcity, finding alternatives to platinum and platinum based precious metal electrocatalysts for oxygen reduction reaction (ORR) has been attracted by many researchers in recent years.1 Non-precious metal based catalysts supported on nitrogen-doped carbon, namely MNC catalysts, have shown reasonable activity and stability in acidic and alkaline media. Among them, Fe based MNC catalysts exhibited highest ORR activity, to date.2,3 In this study, we prepared MNC catalysts by pyrolyzing a mixture of metal salt (iron, cobalt and manganese precursors), nitrogen precursor (melamine, ammonium carbamate and bipyridine) and high surface area carbon (Ketjen Black) in a stainless steel closed reactor with an attached analog pressure gauge. We showed that high pressure pyrolysis (HPP) approach was used to increase the density of nitrogen-based active sites because of the improved equilibrium conditions that occur at high pressures.4-6With HPP method, we obtained up to ~ 400 psi pressure with a mass loading of 3.5 g precursor. Figure 1 shows a representative ORR performance of a Fe-N-C catalyst in acid and alkaline media. Polarization curves obtained at room-temperature in oxygen saturated 0.5 M H2SO4and 0.1 M KOH with a 1200 rpm at a 0.5 mVs1- scan rate. For the synthesis of MNC catalyst, iron and nitrogen content were fixed at 1.2 wt% and 25 wt% respectively. It is obvious that the Fe-N-C exhibits a higher ORR performance in alkaline. In addition to traditional rotating disc electrode studies, square wave-voltammetry technique will be used to probe metal oxidation-reduction couples that provides an estimation of a number of metal-centered complexes in MNC catalysts. Additionally, the effects of the nature and composition of different nitrogen and metal precursors (cobalt and manganese) and carbon supports (carbon nanofibers and mesoporous carbon) on ORR activity will be presented. In order to evaluate MNC catalysts for real-world performance, fuel cell membrane electrode assemblies (MEA) of 5 cm2electrode areas for alkaline and proton exchange membrane fuel cells will also be presented. References M. K. Debe, "Electrocatalyst approaches and challenges for automotive fuel cells," Nature, 486(7401), 43-51 (2012). doi:doi:10.1038/nature11115 G. Wu and P. Zelenay, "Nanostructured nonprecious metal catalysts for oxygen reduction reaction", Acc. Chem. Res., 46, 1878–1889 (2013). doi:10.1021/ar400011z. A. Serov, K. Artyushkova and P. Atanassov, "Fe-N-C oxygen reduction fuel cell catalyst derived from carbendazim: Synthesis, structure, and reactivity", Adv. Energy Mater., 4, 1–7 (2014). doi:10.1002/aenm.201301735. R. Kothandaraman, V. Nallathambi, K. Artyushkova and S. C. Barton, "Non-precious oxygen reduction catalysts prepared by high-pressure pyrolysis for low-temperature fuel cells," Applied Catalysis B: Environmental, 92(1–2), 209-216 (2009). doi:http://dx.doi.org/10.1016/j.apcatb.2009.07.005 V. Nallathambi, N. Leonard, R. Kothandaraman and S. C. Barton, "Nitrogen Precursor Effects in Iron-Nitrogen-Carbon Oxygen Reduction Catalysts," Electrochemical and Solid-State Letters, 14(6), B55-B58 (2011). doi:10.1149/1.356606 S. Ganesan, N. Leonard and S. C. Barton, "Impact of transition metal on nitrogen retention and activity of iron-nitrogen-carbon oxygen reduction catalysts," Physical Chemistry Chemical Physics, 16(10), 4576-4585 (2014). doi:10.1039/c3cp54751e Acknowledgement We gratefully acknowledge financial support from the U.S. Department of Energy (DOE), under Non-PGM Catalyst development effort lead by Northeastern University (Prof. Sanjeev Mukerjee,P.I.). Figure 1

Tuning d-band center of Pt via Pt-Pt5La heterostructure interface as efficient and stable electrocatalyst for oxygen reduction and methanol oxidation reactions in DMFCs

Platinum and platinum-alloy oxygen reduction reaction (ORR) catalysts have been implemented in the prototype, and by now also in commercial fuel cell power systems for automotive applications, successfully meeting the demanding conditions of relatively low temperature (80°C) and high acidity operating conditions of the polymer electrolyte fuel cell (PEFC). However, due to the high price of Pt, the catalyst cost is projected to account for as much as a half of the total PEFC stack cost.1 In this context, the potential replacement of Pt-based catalysts, especially in the fuel cell cathode, with non-precious metal catalysts (NPMCs) is expected to significantly reduce PEFC stack cost. Achieving such a cost benefit is, however, contingent upon reaching sufficient activity and durability with NPMC for the ORR, a requirement that the electrocatalysis research community has been striving to meet. In this presentation, we expand upon the idea of combining nitrogen precursors during NPMC synthesis to improve the performance and performance durability of non-precious metal ORR catalysts. Previously, we introduced a new non-precious metal ORR catalyst derived from two nitrogen precursors with different decomposition temperatures: easily decomposing cyanamide (CM) and more thermally stable polyaniline (PANI).2 In this approach, CM acts as an efficient pore-forming agent that allows for the formation of a highly porous structure, which in particular benefits the high-current response of the resulting electrocatalyst. Compared to the alternative approach utilizing metal-organic frameworks (MOFs),3,4 the use of cyanamide promises a very significant reduction in the NPMC cost, without sacrificing ORR activity. In an effort to further improve the PEFC performance of the two nitrogen-precursor NPMCs, we have explored approaches involving the use of ammonia treatment, the addition of various carbon supports, and inclusion of multiple transition metals. In this presentation, we will summarize the results of these studies, which have involved the extensive use of characterization techniques, such as X-ray photoelectron spectroscopy, scanning electron microscopy, scanning transmission electron microscopy, X-ray diffraction, surface area/pore distribution measurements, and nano-scale X-ray computed tomography. In the conclusion, we will also define critical factors for enhancing the performance of NPMCs in the PEFC cathode and outline directions for future research. Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. Microscopy research supported through a user project at ORNL’s Center for Nanomaterials Sciences, which is an Office of Science User Facility. References Spendelow et al., DOE Fuel Cell Technologies Office, Fuel Cell System Cost-2013, (2013).Hoon T. Chung et al., “Combined nitrogen precursor approach to develop highly active non-precious metal oxygen reduction reaction catalyst.” in preparation.S. Ma et al., “Cobalt Imidazolate Framework as Precursor for Oxygen Reduction Reaction Electrocatalysts,” Chem. Eur. J. 17, 2063 (2011).Proietti et al., “Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells,” Nat. Commun. 2, 416 (2011)

Carbon black supported Pd-Mo bimetallical nanoparticles are prepared by Strong Electrostatic Adsorption (SEA) method. A series of Pd-Mo catalysts were prepared by selective deposition of the [Pd (NH3)4]+2cationion onto the supported MoO3phase. The prepared materials are characterized by N2-BET, powder XRD and STEM. The results showed that good dispersion of bimetallic nanoparticles on the carbon support is observed from STEM images. Oxygen reduction measurements performed by linear sweep voltammetry indicat the good catalytic activity of Pd-Mo by SEA compared to dry impregnation (DI) bimetallic catalysts since the SEA technique produced more well dispersed and intimately designed bimetallic particles after reduction.

Here, the hybrid of NiCo2S4 nanoparticles grown on graphene in situ is first described as an effective bifunctional nonprecious electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the alkaline medium. NiCo2S4@N/S-rGO was synthesized by a one-pot solvothermal strategy using Co(OAc)2, Ni(OAc)2, thiourea, and graphene oxide as precursors and ethylene glycol as the dispersing agent; simultaneously, traces of nitrogen and sulfur were double-doped into the reduced graphene oxide (rGO) in the forms of pyrrolic-N, pyridinic-N, and thiophenic-S, which are often desirable for metal-free ORR catalysts. In comparison with commercial Pt/C catalyst, NiCo2S4@N/S-rGO shows less reduction activity, much better durability, and superior methanol tolerance toward ORR in 0.1 M KOH; it reveals higher activity toward OER in both KOH electrolyte and phosphate buffer at pH 7.0. NiCo2S4@graphene demonstrated excellent overall bicatalytic performance, and importantly, it suggests a novel kind of promising nonprecious bifunctional catalyst in the related renewable energy devices.

Fe/N/C is a promising electrocatalyst for oxygen reduction reaction (ORR). This type catalyst can perform in both acidic and alkaline media. In acidic media, the active sites of Fe/N/C can be corroded; however, in alkaline media, Fe/N/C can exhibit high stability. Therefore, it is promising to explore the applications of Fe/N/C catalyst in alkaline fuel cell. Herein, we will talk about our recent advances in preparation of Fe/N/C catalysts and the application of them in alkaline fuel cells: (1) By using 2-aminothiazole, a molecule containing both N and S atoms, we prepared S co-doped Fe/N/C catalyst with graphene nanosheets. In alkaline solution, this catalyst exhibited high ORR activity with half-wave potential of 0.926 V and mass activity of 0.56 A g-1 @ 1.0 V. Furthermore, the catalyst displayed excellent durability, and only lost 9% of initial activity after 100 h of durability test at 0.80 V. In alkaline anion exchange membrane fuel cell (AEMFC) test, the peak power density could reach 164 mW cm−2. (2) We used the unzipped carbon nanotubes as carbon support and melamine as nitrogen source, and prepared a Fe/N/C catalyst with high ORR activity in alkaline media. The edge sites of unzipped carbon nanotubes play a key role for high catalyst activity. (3) we explored a series of nitrogen source including binary polymer of melamine-terethalaldehyde, ternary polymer of cynuric acid, 2, 4-diamino-6-phenyl-1, 3, 5-triazine, and melamine. N-doped carbon nanotubes with encapsulated Fe nanoparticles were prepared. The catalyst exhibited considerably high ORR activity in alkaline solution. We further tested its performance in alkaline membrane direct ethanol fuel cell, and the peak power density was about 64 mW cm-2. These results demonstrated that Fe/N/C is a promising candidate for ORR electrocatalyst in alkaline fuel cell.

The development of economic and efficient bifunctional electrocatalysts, which can boost both the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) is quite necessary for the evolution of direct methanol fuel cell (DMFC). Herein, a novelty of precious metal‐free, iron, nitrogen and phosphorus‐doped porous carbon (Fe@NPC) was prepared by using cheap and renewable biomass silk as basic nitrogen sources, as an efficient ORR and MOR electrocatalyst with its high nitrogen content (8.16 wt%). The Fe@NPC catalyst exhibits extremely high onset potential (1.125 V vs RHE), current density (−6.089 mA cm −2 ) and methanol tolerance for ORR, even exceeds the corresponding values of Pt/C (1.007 V vs RHE, 5.068 mA cm −2 ). Furthermore, it also served as a good carrier for Pt nanoparticles (NPs). Pt/Fe@NPC shows high catalytic activity and stability for MOR in acidic condition.

Zinc–air batteries (ZABs) have attracted extensive attention due to their remarkable high theoretical energy output. They represent one of the most promising future power sources. However, many barriers restrict their application on a large scale. One of the main challenges is the sluggish rates of the oxygen‐reduction reaction (ORR) and oxygen‐evolution reaction (OER), which govern the discharging and charging processes of the battery, respectively. Here, recent advances related to oxygen electrocatalyst materials for ZABs are discussed. Detailed discussions will focus on unifunctional ORR electrocatalysts and bifunctional ORR and OER electrocatalysts. Pt‐based nanomaterials, as the best ORR electrocatalysts, possess the virtue of high activity, but have the disadvantages of high cost, scarcity, and poor stability. Thus, materials based on transition metals (alloys, metal oxides, metal nitrides, and spinel oxides) and metal‐free materials are widely investigated as nonprecious ORR catalysts owing to their promising catalytic activities. As for bifunctional ORR and OER electrocatalysts, the following two categories are introduced: (i) metal‐based materials, including single metal/metal‐oxides‐based materials and mixed‐metal/metal‐oxides‐based materials; and (ii) metal‐free materials. Finally, perspectives on the continuous research and limitation of the current ZAB technology are provided.

Platinum (Pt)-based nanoparticle metals have received a substantial amount of attention and are the most popular catalysts for direct methanol fuel cell (DMFC). However, the high cost of Pt catalysts, slow kinetic oxidation, and the formation of CO intermediate molecules during the methanol oxidation reaction (MOR) are major challenges associate with single-metal Pt catalysts. Recent studies are focusing on using either Pt alloys, such as Fe, Ni, Co, Rh, Ru, Co, and Sn metals, or carbon support materials to enhance the catalytic performance of Pt. In recent years, Pt and Pt alloy catalysts supported on great potential of carbon materials such as MWCNT, CNF, CNT, CNC, CMS, CNT, CB, and graphene have received remarkable interests due to their significant properties that can contribute to the excellent MOR and DMFC performance. This review paper summaries the development of the above alloys and support materials related to reduce the usage of Pt, improve stability, and better electrocatalytic performance of Pt in DMFC. Finally, discussion of each catalyst and support in terms of morphology, electrocatalytic activity, structural characteristics, and its fuel cell performance are presented.

The direct methanol fuel Cell (DMFC) is a low temperature fuel cell operating at temperatures of 30 to 130°C. The DMFC is powered by a liquid fuel (usually considered essential for transport uses) and is therefore regarded by some as the ideal fuel cell system. In this paper, the DMFC is compared to the hydrogen-fuelled proton exchange membrane fuel Cell (PEMFC) which was discussed in detail in the January and July issues. While a typical DMFC is less efficient than a PEMFC, work to improve its performance with new electrocatalyst materials for utilisation in the membrane electrode assemblies has proved successful. This work is described here and some possible commercial uses for the DMFC are also considered.

Direct methanol fuel cells (DMFC) have several advantages for a broad spectrum of energy density demanding applications, ranging from portable devices to stationary backup power. DMFCs deliver power at high efficiencies compared to internal combustion technologies and uses high energy density methanol as the fuel. However, DMFCs face several challenges, including methanol crossover at the cathode and slow methanol oxidation reaction (MOR) kinetics at the anode that impede its DMFC performance relative to hydrogen polymer electrolyte fuel cells (PEFCs). Since the the anode is a much bigger source of overpotentials than the H2 PEFC, we must carefully consider it’s characteristics in designing the membrane electrode assembly (MEA) and it requires extensive experimental and modeling studies to investigate these hindrances and mitigate their impacts on performance. It is widely known that vapor-fed DMFCs suffer less from methanol crossover. However, a complete understanding of the vapor-fed MOR kinetics in porous electrodes is lacking. When modeling DMFCs it is a common practice to assume that the reaction order of the MOR is zero. However previous studies have shown that the liquid-fed MOR on PtRu/C catalysts has areaction order of 0.5 [1, 2]. However, we found that the reaction order of the vapor-fed MOR can differ from that value. We also investigated the effect of different catalyst loading and the effect of methanol crossover on reaction order.This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under Award Number DE-EE0008440. References S.Lj. Gojkovic´ et al., Electrochimica Acta 48 (2003) 3607/3614D. Chu, S. Gilman, J. Electrochem. Soc. 143 (1996) 1685.

An enhanced non-noble perovskite-based oxygen electrocatalyst for efficient oxygen reduction and evolution reactions

Employing an alternative of the Pt-based electrocatalysts for oxygen reduction reaction (ORR) has become a major interest in the fundamental research of the polymer electrolyte membrane fuel cells (PEMFCs). The carbon-encapsulated metal catalyst, on which O-2 is readily activated by the electrons transferred from the metal to the carbon surface, has recently been demonstrated as a promising strategy to produce robust non-precious metal electrocatalysts. However, the thickness of carbon walls might affect the process of the electron transfer, and subsequently the ORR activity. It is thus vital to explore the influence of the carbon wall thickness on the ORR reactivity for further improvement in designing carbon-encapsulated non-precious metal catalysts for ORR. Herein, we report a novel FeCo alloy catalyst encapsulated in pod-like carbon nanotubes via introducing graphene nanosheets into the raw materials to tailor the carbon wall thickness. The ORR activity of these catalysts increases drastically with the decreased thickness of the carbon walls, which could be attributed to the enhanced adsorption of O-2 on the carbon surface upon decreasing the carbon wall thickness. These findings provide a route for the rational design of high-performance non-precious metal cathode catalysts in PEMFCs.

Proton exchange membrane hydrogen fuel cell (PEMFC) deployment is presently limited by the high material cost of oxygen reduction reaction (ORR) electrocatalysts at the cathode. Developing low-cost, earth-abundant alternatives for use in acidic environments presents a material stability challenge, especially for long-term device operation. Platinum (Pt)-based materials are used in typical commercial PEMFC cathode formulations due to their simultaneous high performance and long-term stability despite the prohibitively high material cost. Developing lower-cost, longer-life catalyst materials based on earth abundant transition metals can accelerate the adoption of PEMFC technology. The impact of potential, current density, and local microenvironment has been shown to cause degradation of non-Pt materials but standard ex situ measurements can only probe surface restructuring and cumulative dissolution after operation. Material-specific degradation mechanisms that are strong functions of the operating conditions cannot be ascertained without in situ quantification of material degradation under PEMFC-relevant conditions. Information about the degradation pathways of these non-precious catalysts can assist with material design for devices; however, most efforts towards developing an in-situ understanding of PEMFC cathode catalyst degradation under operating conditions are generally limited to Pt-based materials. Extensive in situ characterization has revealed Pt-specific mechanistic information about corrosion pathways as well as conditions that exacerbate Pt alloy leaching. It has been hypothesized that the Pt degradation processes are related to oxide formation under potential cycling. With this critical in-situ information, Pt conditioning protocols have been developed to enhance material lifetime; however, investigations on non-precious materials are less common. Insights into non-precious catalyst degradation in PEMFC operating conditions are not as well-established but could be essential in enabling low-cost materials for commercial use. There is a need for more insights into a vast array of materials which necessitates the development of faster, preferably benchtop techniques for degradation analysis.In this work, we use an in situ electrochemical flow cell assembly and coupled it with an inductively coupled plasma-mass spectrometer (ICP-MS) to quantify the loss of material in real time for several non-Pt materials under reaction conditions. The flow cell is fed by a peristaltic pump at 2.5 mL/min of gas-saturated acidic electrolyte for optimal mass transport. The cell has a three-electrode configuration with a compression-mounted metal foil working electrode, silver/silver chloride reference electrode, and Pt counter electrode downstream of other components. We evaluate a variety of transition metals across the spectrum of ORR activity: palladium, silver, nickel, copper, manganese, and cobalt. Each metal is tested under oxygen and nitrogen saturation in five electrolytes of identical pH: perchloric acid, sulfuric acid, nitric acid, hydrochloric acid, and hydrobromic acid. This setup allows for nearly real time measurement of ORR kinetics and in situ element-specific dissolution rates as a function of time. With careful design of experimental conditions and controlled variables, we are able to suggest mechanisms of degradation with specific reference to conditions under which material loss occurred such as high/low potential, oxygen-saturated/nitrogen-saturated electrolyte, or faradaic/non-faradaic current. We find some metals are stable under nitrogen-saturated electrolyte but corrode more under oxygen-saturated electrolyte only while ORR active. Furthermore, other metals are stable in nitrogen saturated electrolyte at all tested electrochemical conditions but unstable under oxygen saturated electrolyte but only outside the ORR range, implying that cathodic faradaic processes might be stabilizing the surface at low potentials. This stabilization where a thermodynamically unstable material (based on its Pourbaix diagram) exhibits greater immunity while performing ORR in certain electrolytes could inform the development stability mechanisms that direct the design of next generation, non-precious PEMFC cathode catalyst materials. With combined in situ catalyst stability analysis in five pH 1 electrolytes, we can recommend specific conditions for optimizing activity and stability based on the potential range of operation and the electrolyte composition/saturation. We can also distill mechanistic insights from these experiments by creating models that validate causes of catalyst degradation. These models are built using “graphical causal modeling”, which is a mathematical field that goes beyond correlative predictions by venturing into causal inference. We hope to expand this framework into other electrocatalytic reactions of interest. The in-situ ICP-MS electrochemical flow cell setup exhibits promise for accelerating materials stability analysis for enabling next generation Pt-free ORR electrocatalysts through practical experiments and direct comparability to established electrochemical techniques.

Understanding of less-noble-metal (M) dissolution from Pt-alloy-based oxygen reduction reaction (ORR) electrocatalysts, as well its interaction with Pt surface, is crucial for maximizing their performance. In pursuing this goal, two ORR electrocatalysts—a benchmark Pt–Co/C and an in-house designed Pt–Cu/C materials—are investigated. Both are characterized with a range of standard techniques, such as X-ray diffraction (XRD), transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDX) and thin film-rotating disc electrode (TF-RDE) measurements. A special focus is put on combining the latter with a highly sensitive electrochemical flow cell (EFC) online connected to inductively coupled plasma mass spectrometry (ICP-MS) measurements. A combination of standard and novel techniques provides unprecedented insights into the dissolution behavior and dynamics of metals, as well as their subsequent surface interactions and effects on the electrochemical performance. A special focus is devoted to the significance of electrocatalyst activation protocols and their effects on metal dissolution. Distinct differences in the behavior of Pt–Cu and Pt–Co alloys are revealed, which will help develop proper protocols for practical implementation of Pt-alloy electrocatalysts.